delete init part when keeping trivial loop (#1031)

include/tvm/operation.h

include/tvm/schedule_pass.h

src/api/api_schedule.cc

src/codegen/build_module.cc

src/op/compute_op.cc

src/op/compute_op.h

src/op/cross_thread_reduction.cc

src/op/extern_op.cc

src/op/op_util.cc

src/op/op_util.h

src/op/placeholder_op.cc

src/op/scan_op.cc

src/op/tensorize.cc

src/schedule/schedule_ops.cc